1. Field of the Invention
The present invention relates to nickel powders and to methods for producing such powders, as well as intermediate products and devices fabricated using the powders. In particular, the present invention is directed to powder batches of nickel particles with a small average particle size, well controlled particle size distribution, spherical morphology and high crystallinity.
2. Description of Related Art
Many product applications require metal-containing powders with one or more of the following properties: high purity; high crystallinity; small average particle size; narrow particle size distribution; spherical particle morphology; controlled surface chemistry; reduced agglomeration of particles; and high density (low porosity). Examples of metal powders requiring such characteristics include, but are not limited to, those useful in microelectronic applications, such as for multilayer ceramic capacitors (MLCC's), multichip modules and other devices, including flat panel displays.
Electronic devices such as capacitors, and in particular MLCC's, have traditionally incorporated electrodes fabricated from noble metals such as palladium. MLCC's are fabricated by stacking alternate layers of a ceramic dielectric and a conductive metal and then sintering (heating) the stack to densify the layers and obtain a monolithic device. Most ceramic dielectric compounds are oxides that must be sintered at an elevated temperature in an oxygen-containing atmosphere to avoid reduction of the ceramic and the loss of the dielectric properties. Noble metals such as palladium advantageously resist oxidation under these conditions. However, noble metals are relatively expensive and significantly increase the fabrication cost of such devices. Therefore, it would be advantageous to utilize less costly base metals for such applications. Base metals such as nickel are generally at least an order of magnitude less costly than noble metals. But most base metals have a tendency to oxidize when held in an oxygen-containing atmosphere at elevated sintering temperatures, thereby ruining the electrical properties of the metal and creating other problems in the device, such as delamination of the stacked layers.
There have been attempts in the art to address some of the problems associated with using base metals in such microelectronic devices. U.S. Pat. No. 3,902,102 by Burn discloses a ceramic capacitor utilizing electrodes fabricated from nickel or copper powder having a particle size of less than about 325 mesh (44 .mu.m). The metal is protected from oxidation during sintering of the capacitor by the addition of a barium borate glass to the thick film paste composition used to apply the electrode.
U.S. Pat. Nos. 3,966,463, 4,010,025 and 4,036,634, all by Fraioli et al., disclose an oxidation resistant powder which includes gold or nickel metal and small amounts of a co-nucleated oxide, such as titania or zirconia, formed by co-nucleation and precipitation from an ammoniacal solution with sodium bisulfite. The powder has good tap density, which improves the rheological properties of pastes made from the powder. It is disclosed that nickel oxidizes to nickel oxide in air between 350.degree. C. and 700.degree. C. and that a high surface area nickel powder can oxidize at room temperature. The nickel/zirconia powder with about 2 weight percent zirconia is able to withstand one hour in air at 450.degree. C. with no measurable weight gain due to oxidation.
U.S. Pat. No. 4,115,493 by Sakabe et al. discloses a ceramic dielectric for an MLCC that can be sintered in a reducing atmosphere, and therefore permits nickel electrodes to be utilized. It is disclosed that a paste, including nickel powder having an average particle size of about 1 .mu.m, can be screened onto the ceramic dielectric to form the MLCC structure.
U.S. Pat. No. 4,122,232 by Kuo discloses a thick film paste for forming a conductor, including 50 to 80 weight percent nickel powder and 5 to 20 weight percent boron powder. It is disclosed that the boron powder advantageously reduces oxidation of the nickel powder. In one example, nickel powder having a particle size between about 2.9 .mu.m and 3.6 .mu.m is utilized in the thick film paste.
U.S. Pat. No. 4,223,369 by Burn discloses a zirconate dielectric composition including boron that can be sintered in a reducing atmosphere. The use of a reducing atmosphere during sintering allows nickel electrodes to be used. U.S. Pat. No. 4,700,264 by Kishi et al. also discloses a dielectric that can be sintered in a reducing atmosphere. Nickel electrodes are utilized in the ceramic capacitor and it is disclosed that the nickel powder has an average particle size of about 1.5 .mu.m.
U.S. Pat. No. 4,954,926 by Pepin discloses a thick film paste composition including organometallics that advantageously reduce delamination defects in the MLCC. The metal powder can include nickel powder.
It can be seen from the foregoing that there are significant advantages to using base metals, such as nickel, for the formation of electrodes in microelectronic applications or other devices such as flat panel displays. Nickel powders are less expensive than noble metals and provide good conductivity. Nickel metal also resists leaching (degradation) during soldering.
Other uses for fine nickel metal powders include their use to form dispersion strengthened alloys or for porous barriers for the gaseous phase separation of uranium isotopes. Such applications are disclosed in U.S. Pat. No. 3,748,118 by Montino et al. U.S. Pat. No. 3,850,612 by Montino et al. also discloses that spherical nickel metal powders can be advantageously utilized for slip casting of metal components because such powders yield green castings of greater uniformity and density. The powders are also useful as catalysts where the high surface area accelerates chemical reactions. The powders can also be used to fabricate porous electrodes and filters or membranes having controlled permeability. U.S. Pat. No. 4,578,114 by Rangaswamy et al. also discloses the use of composite nickel powders as a thermal spray powder for the deposition of a thermal spray coating onto a substrate. A similar application is also disclosed in U.S. Pat. No. 5,063,021 by Anand et al. Each of these U.S. patents is incorporated herein by reference in its entirety.
Different methods have been proposed to produce nickel metal powders. U.S. Pat. No. 3,711,274 by Montino et al. discloses a process for preparing spherical, sub-micron nickel powder by heating a suspension of bis-acrylonitrile-nickel in methanol to produce nickel particles. It is disclosed that the nickel retains up to 15 percent organic impurities and can be purified by hydrogenating the powder at elevated temperatures. The average particle size is about 57 nanometers.
U.S. Pat. No. 3,748,118 by Montino et al. discloses a process for producing spherical nickel powder by heating a hydroalcoholic suspension of a nickel compound under hydrogen pressure. The average particle size of the nickel is from about 0.07 .mu.m to about 2 .mu.m. U.S. Pat. No. 3,850,612 by Montino et al. discloses a similar process for producing nickel powder having an average particle size of from about 0.03 .mu.m to about 0.7 .mu.m.
The article entitled "Preparing Monodispersed Metal Powders in Micrometer and Submicrometer Sizes by the Polyol Process" by Fievet et al. (MRS Bulletin, December, 1989) discloses the preparation of nickel metal powders by the reduction of nickel hydroxide in ethylene glycol. It is disclosed that the nickel powders have a small size and a narrow size distribution. As with most liquid preparation routes, the particles have low crystallinity (i.e. a small average crystallite size). Viau et al., in an article entitled "Preparation and Microwave Characterization of Spherical and Monodisperse Co.sub.20 Ni.sub.80 Particles" (J. Appl. Phys., 76, (10), 1994), disclose cobalt-nickel alloy particles produced using a similar process.
Spray pyrolysis is not in common use for the production of nickel powders containing small particles, such as those having an average particle size of not greater than about 5 .mu.m. This is believed to be due to the high processing costs and low production rates typically associated with spray pyrolysis. Further, spray pyrolysis methods often produce hollow particles that are not sufficiently densified for most applications. Generally, spray pyrolysis methods include the generation of liquid droplets wherein the liquid is a solution of a particle precursor. The droplets are then heated to evaporate the liquid, react the precursors, and form solid particles.
The article entitled "Preparation of Fine Ni Particles by the Spray-Pyrolysis Technique and Their Film Forming Properties in the Thick Film Method," by Nagashima et al. (Journal of Materials Research, Vol. 5, No. 12, December 1990) discloses the formation of nickel metal particles by spray pyrolysis and the use of those particles for thick film pastes. Nickel particles are formed from nickel nitrate (Ni(NO.sub.3).sub.2) and nickel chloride (NiCl.sub.2) solutions. The solutions were atomized by an ultrasonic atomizer and an N.sub.2 /H.sub.2 carrier gas was utilized to carry the droplets to a heated reaction zone. The reaction temperature was varied from 500.degree. C. to 1600.degree. C.
The article entitled "Preparation of Nickel Submicron Powder by Ultrasonic Spray Pyrolysis" by Stopic et al. (The International Journal of Powder Metallurgy Vol. 32, No. 1, 1996) discloses the formation of nickel powder by spray pyrolysis. Powders were formed at temperatures of 900.degree. C. to 1000.degree. C. under a reducing atmosphere. The authors state that the powders had a spherical morphology and were substantially crystalline.
There remains a need for nickel powders having a small particle size, narrow size distribution, high crystallinity (large crystals) and spherical morphology. It would be particularly advantageous if such powders could be produced in large quantities on a substantially continuous basis.